This disclosure relates generally to optical tracking systems, and in particular but not exclusively, to optical tracking systems for use in optical communication systems.
With the increasing popularity of wide area networks, such as the Internet and/or World Wide Web, network growth and traffic have exploded in recent years. Network users continue to demand faster networks, and as network demands continue to increase, existing network infrastructures and technologies are reaching their limits.
An alternative to existing hardwire or fiber network solutions is the use of wireless optical telecommunications technology. Wireless optical telecommunications utilize beams of light, such as lasers, as optical communications signals, and therefore do not require the routing of cables or fibers between locations. Data or information is encoded into a beam of light, and then transmitted through free space from a transmitter to a receiver. The receiver includes a communications detector (including a demodulator or decoder) to extract the data or information from the optical signals.
For point-to-point free space laser communications, the use of narrow optical beams provides several advantages, including data security, high customer density, and high directivity. High directivity makes the achievement of high data rates and high link availability easier, due to higher signal levels at a receiver. In order to take full advantage of this directivity, some form of tracking is often necessary to keep the antennas of a transmitter and of the receiver properly pointed at each other. For example, a transmitted optical beam with a one milli-radian divergence has a spot diameter at the receiver of about one meter at a one kilometer range. Thus, movement of the transmitter or receiver by even a small fraction of the divergence (or field-of-view) could compromise the link unless active tracking is employed.
In accordance with aspects of the present invention, an optical tracking system for use in an optical receiver of an optical communication system is provided. In one aspect, the optical tracking system includes a first focus unit, a tracker, and an optical fiber having an angled tip. The first focus unit receives an optical signal and focuses the optical signal on the tip of the fiber. The fiber is connected to a communications detector of the optical communication system. In addition, the angled tip of the fiber reflects a portion of the focused optical signal to the tracker. The tracker processes the reflected portion of the optical signal to correct for any misalignment between the optical signal and the optical receiver. This aspect of the invention advantageously eliminates the need for a beam splitter, which are typically required in known optical tracking systems.
In another aspect of the present invention, the optical tracking system further includes a second focus unit to receive the reflected portion of the optical signal and focus it for use by the tracker. In one embodiment, the second focus unit can include a focusing lens and a wedge. The wedge is used to steer the reflected portion of the optical signal to the focusing lens, which then focuses the reflected portion of the beam on an optical detector of the tracker. This embodiment is advantageously used in systems in which the fiber can be moved in an axial direction. In one embodiment, the optical detector includes a quad cell detector circuit.
In another aspect of the present invention, a coating is added to the surface of the tip of the angle-tipped fiber. The coating is partially reflective with respect to the optical signal, allowing a majority of the optical signal to pass through to be propagated by the fiber. This aspect can be advantageous in that when there is misalignment between the optical signal and the optical receiver, the optical signal would tend to illuminate the cladding region of the fiber rather than the core. The reflective coating then increases the power of the portion of the optical signal received by the tracker.